The current invention relates generally to position tracking and machine control systems, and in particular to a combination laser detector and global navigation satellite antenna useful in position tracking and machine control systems.
In prior art related to position tracking and machine control systems, global navigation satellite systems, like GPS and GLONASS have been used extensively to determine position coordinates facilitating automated control of a mobile unit. In the future, the European GALILEO system will have similar capabilities. An autonomous navigational system that includes a satellite receiver and a navigational computer can achieve a 10-meter level of accuracy in the position determination of a mobile unit using solely the satellite signals. Differential navigational systems that utilize differential corrections in addition to the satellite signals can determine the positional information to within a meter range of accuracy. Real-time kinematic (RTK) navigational systems that are capable of utilizing in real time not only code but also carrier information transmitted from the satellites can achieve centimeter level accuracy in the position determination of a mobile unit.
However, a level of accuracy under a centimeter is still beyond the reach of typical satellite-based navigational systems. In an attempt to achieve very high accuracy, prior art solutions have been to use rotating laser-based systems to define the plane level (Z-plane) to millimeter level accuracy. However, these prior art laser-based systems cannot be used for the purposes of three dimensional navigation of mobile objects because they are configured to determine only one (Z) coordinate of the mobile object with great accuracy. Improvements are therefore still needed in the art.
Complicating the effort to achieve millimeter level accuracy for all three coordinate positions (x, y, z) of a mobile unit is that a global navigation satellite receiver is typically designed to compute the location of its antenna. This means that in order to capture the location of an object of interest, an offset from the antenna's location must be applied to determine the horizontal (x, y) coordinates of the object of interest. An additional offset must also be applied to the location of the object of interest if a laser receiver is used to determine its vertical (z) coordinate. Typically, these offsets are manually entered into the control system and are based on manually measuring the locations of the antenna and laser receivers relative to the object of interest. For example, operators must manually measure the locations of the satellite antenna and laser receivers mounted separately on an excavator, to the excavator's bucket tip and enter those offsets into the excavator's control system. However, knowing precisely the offsets to be used for the satellite antenna and the laser receivers is an essential part of doing the most precise surveying possible. Manually taking and entering such measurements may introduce significant errors in the positional (x, y, z) computations performed by the control system in order to determine the location of an object of interest.
Additional errors may further creep into the positional (x, y, z) computation as global navigation satellite antennas are calibrated under somewhat ideal conditions. Typically, in situ calibrations are performed at a consistent height, and on flat terrain with no reflectors, other than the ground, that may cause unwanted multipath reflections leading to azimuthal asymmetries. Accordingly, for use at a construction site, the conditions under which these calibrations were determined are a somewhat unreasonable approximation of the actual conditions under which antennas are used at a construction site. Under ideal circumstances, every antenna would be individually calibrated at its own site. While this is possible and might be accomplished for permanent global navigation satellite tracking sites, it is impractical for sites that are only infrequently and briefly occupied, such as a plot of land being worked.